This invention relates to electric machines in general and more particularly to a device for cooling a superconducting field winding in the rotor of an electric machine, especially a turbo-generator.
A device for cooling a superconducting field winding in the rotor of an electric machine which includes at least one coolant space containing, in the operating condition, a vaporous and a liquid phase of cryogenic coolant fed in from outside the rotor, and which has coolant paths going through the field winding which are connected to the liquid space of the coolant space occupied by the liquid phase is known. At least one coolant exhaust gas line leads to outside the rotor and is connected to the vapor space of the coolant space occupied by the gaseous phase. The exhaust gas line is connected thermally to an end piece at the end face of a torque transmitting hollow cylindrical rotor body part. At least one coolant line extends along a damper shield which surrounds the field winding and is to be deep cooled, in the region of the superconducting field winding at the torque-transmitting rotor body part and is thermally insulated from the field winding by a hollow cylindrical body of solid insulating material. Such a cooling device is described in the report of the Electric Power Research Institute, U.S.A.; "EPRI EL-577, Project 429 -1, Final Report", November 1977, pages 3-258 to 3-270.
The superconducting field winding of a generator must be kept at a sufficiently low temperature during the operation of the machine that its superconductors cannot make a transition into the normally conducting state. Generally, therefore, cooling of the winding with liquid helium used as the cryogenic coolant is provided, where the dissipation heat produced in the superconductors and the heat introduced from the outside via the end pieces at the end face of the torque transmitting parts of the rotor body lead to partial evaporation of coolant. The heat flow introduced into the low temperature region of the machine can be reduced considerably however by using the principle of counter flow cooling, if the evaporated, but still cold, coolant exhaust gas is conducted in good thermal contact along the torque transmitting end pieces of these rotor body parts, absorbing there a large part of the inflowing heat. The coolant exhaust gas is warmed up in the process and leaves the rotor at a connecting head as warm gas.
In the superconducting field winding itself, dissipation heat is produced among other things if the superconductors are subjected to a magnetic aternating field. This takes place in the rotor of a generator, for instance, if not all current phases of the stator winding are loaded uniformly and further, after progressive short-circuit switching or in the case of oscillations when the rotor speed temporarily does not agree completely with the predetermined network frequency. In order to keep the alternating field amplitudes at the superconductors small under such transient operating conditions, the superconducting field winding can be surrounded by one or more damper shields which shield the magnetic field and at the same time damp oscillations. For shielding low frequency fields, damper shields of thermally and electrically highly conductive material, held at a low temperature, are advantageous. For, practically no dissipation heat is produced in a damper shield in steady-state generator operation. Only during the relatively short transient operating cases, especially in the case of oscillations, is a very large amount of dissipation heat temporarily released. Contrary to the superconductors of the field winding, the temperature of which must remain below the transition temperature of its superconductive material even during transient operating conditions, the temperature of the cold damper may rise more, for instance, to 20 to 30 K.
In the turbo-generator known from the cited EPRI report with a cooling device of the type detailed above, a superconducting field winding surrounded by such a cold damper shield is provided. This generator contains a central coolant space which is subdivided into a liquid space away from the axis and a vapor space near the axis by a liquid and gaseous phase of the coolant helium. Liquid helium is fed into it from the outside. In cooling the field winding with liquid helium taken from the liquid space, a so-called self-pumping effect is utilized such as is described, for instance, in the dissertation by A. Bejan: "Improved Thermal Design of the Cryogenic Cooling System for a Superconducting Synchronous Generator", PhD. Thesis, Massachusetts Institute of Technology, U.S.A., Dec. 1974, pgs. 148 to 159. The damper shield of the machine known from the EPRI report is fastened to the outside of a torque transmitting support cylinder of the rotor body, in which individual canals are provided which serve as damper cooling canals. For this indirect cooling of the damper shield, liquid helium which is derived from the coolant paths leading to the superconducting field winding is used. The flow in the damper cooling canal is brought about by a so-called thermo-siphon effect: for heat generated in the cold damper shield travels through the support cylinder, gets into the helium contained in its cooling canal and excites convection there. Due to this convection, the warmed helium flows back into the helium bath provided at the center of the rotor, where a corresponding amount of liquid helium evaporates. While the liquid phase of this helium bath is separated from the liquid space of the central coolant space, the evaporation space of this bath forms part of the vapor space of the central coolant space. From this vapor space, evaporated helium is discharged from the rotor via exhaust gas lines, which are in thermal connection with the end pieces of the torque transmitting rotor body parts at the end faces. In the process, the coolant exhaust gas is brought approximately to room temperature and the torque transmitting end pieces are cooled down in accordance with the counterflow principle.
While it is ensured in this known cooling device that the heat from the cold damper shield does not get directly into the region of the superconducting field winding because the damper cooling canals are thermally insulated from the winding cooling canals by a solid hollow cylindrical insulating body, relatively large amounts of coolant exhaust gas appear in a very short time in this cooling device, in the case of transient operation, due to a temperature rise of the damper shield connected therewith because of the good heat transfer from the support cylinder into the helium contained in its cooling canal. This gas first gets into the vapor space of the central coolant space. A fast discharge of these amounts of exhaust gas through the exhaust gas lines connected to the torque transmitting end pieces of the rotor body part is difficult, however, since these lines have a relatively small flow cross section. The pressure in the central coolant space then rises accordingly. As a consequence, the saturation temperature of the helium used for cooling the superconducting field winding in the liquid space also increases and thereby, the temperature of the winding itself. This cooling device, in which a relatively large amount of liquid coolant is used up for an effective cooling of the damper shield can therefore be used only for generators of low power rating.
Another turbo-generator with a superconducting field winding and two damper shields to be cooled to a low temperature is described in the report by M. T. Bown et al entitled "Rotor Cooling System for 10-MVA Superconducting Generator" from: Proceedings of the 1979 Cryogenic Engineering Conference, Madison, Wisconsin, U.S.A. Aug. 21, to 24, 1979, paper IC 9. In the cooling device of this machine, the cooling of the damper shields and the cooling of the winding are largely separated form each other. Accordingly a separate coolant supply chamber is provided, in addition to the central coolant space for receiving the coolant required for cooling the superconducting field winding. The liquid helium required for cooling the damper shields is stored in this separate coolant supply chamber. In addition, a phase separator for separating gaseous helium components is provided in the coolant loop for cooling the damper shields. If heat is now suddenly liberated in the damper shields, a coolant circulation develops due to a thermo siphon effect from the helium bath contained in the coolant supply chamber through the first damper shield to the phase separator and subsequently through the second damper shield back to the helium bath. Through the heat input, helium vapor is generated in the helium bath of the coolant supply chamber and in the phase separator. The helium vapor can escape through special gas outlet openings and furthermore, through an emergency exit provided in the separator. A return valve in the connection between the coolant supply chamber and the coolant space containing the cooling bath for the winding prevents a pressure rise in the coolant supply chamber from reacting on the coolant bath of the winding. This valve further makes replenishing the coolant supply chamber with liquid helium possible. Exhaust gas from the coolant supply chamber and the phase separator is used for cooling the end pieces of the torque trasmitting rotor body parts of this machine. In this cooling device, the design of which is rather complicated, the difficulty exists, however, that the helium vapor which is formed in the coolant bath of the winding cannot escape as long as the return valve of the coolant supply chamber is shut. However, the pressure and thereby, the temperature of the coolant provided for cooling the field winding can then increase. In addition, a relatively large amount of liquid helium is required for effective cooling of the damper shields.
It is therefore an object of the present invention to simplify a cooling device of the type mentioned at the outset and to lower its coolant consumption. It should nevertheless be possible to discharge the coolant vapor occuring due to losses in the damper shield from the rotor without substantial warming of the liquid coolant that must be provided for cooling the field winding.